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blkcg: consolidate blkg creation in blkcg_bio_issue_check()

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blkg (blkcg_gq) currently is created by blkcg policies invoking
blkg_lookup_create() which ends up repeating about the same code in
different policies.  Theoretically, this can avoid the overhead of
looking and/or creating blkg's if blkcg is enabled but no policy is in
use; however, the cost of blkg lookup / creation is very low
especially if only the root blkcg is in use which is highly likely if
no blkcg policy is in active use - it boils down to a single very
predictable conditional and surrounding RCU protection.

This patch consolidates blkg creation to a new function
blkcg_bio_issue_check() which is called during bio issue from
generic_make_request_checks().  blkcg_bio_issue_check() is now the
only function which tries to create missing blkg's.  The subsequent
policy and request_list operations just perform blkg_lookup() and if
missing falls back to the root.

* blk_get_rl() no longer tries to create blkg.  It uses blkg_lookup()
  instead of blkg_lookup_create().

* blk_throtl_bio() is now called from blkcg_bio_issue_check() with rcu
  read locked and blkg already looked up.  Both throtl_lookup_tg() and
  throtl_lookup_create_tg() are dropped.

* cfq is similarly updated.  cfq_lookup_create_cfqg() is replaced with
  cfq_lookup_cfqg()which uses blkg_lookup().

This consolidates blkg handling and avoids unnecessary blkg creation
retries under memory pressure.  In addition, this provides a common
bio entry point into blkcg where things like common accounting can be
performed.

v2: Build fixes for !CONFIG_CFQ_GROUP_IOSCHED and
    !CONFIG_BLK_DEV_THROTTLING.

Signed-off-by: Tejun Heo <[email protected]>
Cc: Vivek Goyal <[email protected]>
Cc: Arianna Avanzini <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Tejun Heo 2015-08-18 14:55:20 -07:00 • committed by Jens Axboe
commit ae11889636
6 changed files with 57 additions and 99 deletions
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2
block/blk-cgroup.c
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@ -153,6 +153,7 @@ struct blkcg_gq *blkg_lookup_slowpath(struct blkcg *blkcg,
return NULL;
}
EXPORT_SYMBOL_GPL(blkg_lookup_slowpath);
/*
* If @new_blkg is %NULL, this function tries to allocate a new one as
@ -295,7 +296,6 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
return blkg;
}
}
EXPORT_SYMBOL_GPL(blkg_lookup_create);
static void blkg_destroy(struct blkcg_gq *blkg)
{